Antenna



Aug. 1, 1939.

A. ALFORD' 2,167,735

ANTENNA 7 Filed March 17, 1936 2 Sheets-Sheet 1 nnv/x MAM) INVENTORANDREW ALFORD ATTORNEY Au .1,1939. L R 2,167,735

ANTENNA Filed March 17, 1936 2 Sheets-Sheet 2 lNVENTOR 1 ANDREW ALFORDBY g I ATTO RN EY til Patented Aug. 1, 1939 UNITED STATES PATENT OFFICEANTENNA Application March 1'7, 1936, Serial No. 69,292

7 Claims.

This invention relates to antenna structures for radio communication andpertains more particularly to lei-directional antennas adapted for shortwave communication.

An object of the present invention is a bidirectional antenna which hasa nearly constant input impedance.

Another object of the present invention is a pi-directional antennawhich may be conveniently installed on a ship, and which will requireonly relatively simple arrangements for efliciently transferring thepower from the antenna to the transmission line, and from there to thereceiver when this antenna is used for receiving, and from thetransmitter to the transmission line and from there to the antenna whenthe latter is used for transmitting.

.In the past, two types of long wire antennas have been used; theterminated antennas and the open-ended antennas. The terminated antennashave always been unidirectional, while the open-ended antennas havealways been bidirectional, unless provided with suitable refiectors.

The terminated antennas, when properly terminated, have input impedanceswhich remain nearly constant as the frequency is varied and for thisreason they are well suited for use at a number of differentfrequencies.

The open-ended antennas have input impedances which vary with frequencywithin rather wide limits and consequently they are not suited for useat a number of different frequencies except when used in conjunctionwith relatively complicated matching devices.

Thus, generally speaking, the terminated antennas are superior to theopen-ended antennas in those instances in which the constancy of inputimpedance is one of the primary requirements.

Now, there are applications which require bidirectional rather thanuni-directional antennas. For instance, aboard a ship, which sails backand forth approximately along the great circle between two ports,antennas should preferably be, in certain instances bi-directionalrather than uni-directional, so that the ship may communicate with bothports.

Under these circumstances, in the past, the open-ended type antennaswould have been used, since the terminated antennas of the prior artwere uni-directional unless provided with objectionable switchingarrangements and transmission lines extending from the transmitter orreceiver, as the case may be, 1' 100th n s Of the antenna. Since inpractice it is often found that a transmission line from the translatingdevice to the far end of the antenna cannot be installed, particularlyon a ship, it is clear that in the past it would have been necessary touse bi-directional antennas having end impedances which vary withfrequency over a considerable range, an undesirable characteristic wherecommunication is to be carried out at several different frequencies.

In accordance with the present invention it is possible to construct abi-directional antenna which has a nearly constant near-end impedancewithin a considerable range of frequencies.

The above mentioned and further objects and advantages of my invention,and the manner of attaining them will be more fully explained in thefollowing description taken in conjunction with the drawings.

In the drawings Fig. 1 shows a simple form of antenna constructed inaccordance with my invention.

Fig. 2 shows a modified form of antenna embodying my invention.

Fig. 3 shows the antenna of Fig. 1 installed on a ship in the immediatevicinity of a stay and means for detuning the stay, and

Figs. 4 and 5 show other arrangements for detuning stays.

In Fig. 1, reference numerals H and i2 indicate two inclined wiresjoined together at their far ends by jumper l3 and connected to aconcentric tube transmission line l5 through an aperiodic couplingdevice It which matches the impedance of the antenna to the surgeimpedance of the transmission line l5. A resistance It the value ofwhich is equal to the surge impedance of the wire I2 is connectedbetween the lower end of wire l2 and the ground. Seventeen (ll) is atranslating device, that is, either a receiver or a transmitter, 18 is atower, and i9, 20, 2| are insulators.

The operation of the antenna shown in Fig. 1 may be described asfollows: electric currents generated in transmitter ll proceed throughtransmission line [5 into matching device M. Emerging from l4 thesecurrents proceed in the form of a travelling wave toward jumper I3.After passing through jumper it these waves proceed along wire l2 towardthe terminal resistor I6. When the value of resistor I6 is properlychosen, that is, when it is equal to the surge impedance of wire l2, thewaves travelling along 12 are not reflected at It.

For this reason there is no primary reflected wave on 12 which,travelling through l3, could cause standing waves along I I.Consequently wire II acts in approximately the same manner as though itwere terminated into its surge impedance at the point of junction withjumper I3.

In operation wire II induces currents in wire I2, and vice versa wire I2induces currents in wire II, this results in the formation of a certainamount of standing waves on both wires. The amount of these standingwaves, however, is much less than one might expect. Thus, for example, Ifind by actual experiment at frequencies of the order of 10 to 20megacycles that when wires II and I2 are the order of 1 /2 wave-lengthslong and are placed a foot or two from each other the amount ofreflected wave on portion I 0 of wire II adjacent to the coupling deviceI4 is only about 10%. This result is somewhat surprising particularly inview of the fact that one might expect some reflected waves due toreflection at the junction points of I3 with wires II and I2, inaddition to the reflected waves produced by the mutual inter-action ofwires II and I2.

Because of the relative absence of reflected waves in portion II] ofwire II, the input impedance of the antenna at I4 is nearly independentof the frequency and consequently matching device I 4, capable ofmatching two fixed impedances, namely the impedance of the antenna andthe surge impedance of I5 at various frequencies within a certain band,is all that is required for properly terminating transmission line I5.When line I5 is so terminated transmitter I! will work into nearly thesame impedance at all frequencies within the band in which matchingdevice I4 is operative.

When it is desired to carry on communication between two pointsseparated by a distance greater than about 400 miles and when ahorizontal reflecting surface such as grounder sea is located at adistance of a fraction of a wavelength below the lower end of theantenna it is found that the following table gives approximately therelation between the length of each of the wires II and I 21/). inwavelengths and the angle 0 to which they should be inclined to thehorizontal.

Degrees The values of 0 for Z/x not given in the table may beinterpolated or extrapolated.

In a few cases when the lower end of the antenna is located at aconsiderable height above the reflecting surface it may be found thatthe reflected wave is detrimentally out of phase with the direct wave,in which case the height, or angle of inclination or other constants ofthe antenna may be adjusted to correct this difliculty, in a mannerwhich will be obvious to one skilled in the art.

Fig. 2 shows a modification of the antenna shown inFig. 1. Thedifference between these antennas of Figs. 1 and 2 is that wires I I andI2 in the antenna of Fig. 2 do not end near the tower but continuedownward on the other side of the tower. The various parts and deviceswhich were already described in connection with Fig. 1 have beendesignated by the same numbers inFig. 2 in order to save repetition. InFig.

,lengthaswires III and II2.

2 wires III and I I2 are continuations of wires II and I2. Jumper I3 inFig. 2 connects the ends of I I I and II2 instead of the ends of I I andI2.

The operation of the antenna of Fig. 2 is quite similar to the operationof the antenna in Fig. 1. The gain which is obtainable with the antennaof Fig. 2 is somewhat greater than that obtainable with the simplerstructure of Fig. 1. The interaction between wires II, I2, III, and II2is also somewhat greater than the interaction between wires of thesimpler structure of Fig. 1.

For example, when wires II, I2, I I I, II2 are made each about twowavelengths long and the distance between II and I2, and III and II2 is6 feet, I find that any variation in the antenna impedance at I4 is buta small percentage of the variation which would occur with an open-endedantenna.

The angle at which wires II and I2 as well as the angle which wires IIIand II 2 should make to the horizontal may be determined from the table.Wires II and I2 need not be of the same In some cases, however, it maybe necessary to install phase correcting means at the point of junctionof wires II and III and wires I2 and II2. The operation of such phasecorrecting means has been described in my copending application, SerialNo. 18,995, filed April 11, 1935.

Fig. 3-shows the antenna which was discussed in connection with Fig. 1installed aboard a ship. In this figure numbers III to 2I refer to thesame parts and apparatus which have already been described in connectionwith Fig. 1.

In practice, when an antenna is installed aboard a ship it is oftenfound that stays, low frequency antennas and other wires or metal mastsmay interfere with the proper operation of the antenna. Thus, forexample, stay 30 in Fig. 3 may sometimes interferewith the proper actionof the main radiating wires II and I2 of the antenna.

The mechanism of the interfering action is briefly as follows: theradiating portions of the antenna located in the vicinity of a stayproduce an electric field which has a component along the stay. Thiscomponent of the electric field produces an electromotive force whichsets up currents in the stay. The magnitude of the current so producedin the stay depends on two factors; namely, (1) the magnitude ofelectromotive force along the stay and (2) the self impedance of thestay. When high frequency currents flow through a long conductor someenergy is always radiated into space. In this respect a stay does notdiffer from an antenna wire. Since the phase of the radiation from astay may have any relation whatsoever to the phase of radiation from theinducing antenna, this parasitic radiation from the stay may eitherincrease or decrease the field produced by the antenna in a givendirection. The more sharply directional is the inducing antenna, thegreater is the probability that a stay or any other haphazardly placedwires in the vicinity of the antenna will decrease, rather thanincrease, the total radiation in the desired direction.

In addition to being able to scatter energy by radiation a stay maydissipate a substantial portion of the energy by turning it into heat.This occurs when the internal resistance of the stay is fairly large incomparison with its radiation resistance.

When the internal resistance of a stay is very high so that the majorportion of the energy which it picks from the antenna is dissipated asheat, the small radiated portion does not affect the directionalcharacteristic of the antenna as seriously as in the case when theinternal losses in the stay are relatively small and when most of thepicked up power is radiated. Thus, a high resistance stay tends todecrease the total radiated power without distorting the directionalcharacteristic of the antenna while a low resistance stay tends todistort the directional characteristic without changing the totalradiated power.

Since both the power radiated from, as well as the power dissipated in astay, is proportional to the square of the current which is induced init, it is clear that the effect of a stay on an antenna may becontrolled by controlling the induced current. It has already beenpointed out that the magnitude of this current depends among otherthings on the self impedance of the stay. By making the self impedanceof a stay as large as possible the induced current in it may be reducedto a minimum.

If a stay may be cut and insulators inserted at frequent intervals, theself impedance of each section may be made so high that the stay becomesentirely inactive either as a radiator or a dissipator. This procedureis well known in prior art.

When, however, a stay can not be cut and broken up by insulators theproblem of eliminating the effect of the stay on an antenna is much morediiiicult. In the first place, since the self impedance of a stay of agiven length varies with frequency and at a given frequency may have anyvalue lying between the radiation resistance, which may be of 100 ohms,up to 10 or 15 times the radiation resistance, it is clear that, ingeneral, a given stay may not interfere with an antenna at one frequencyand still cause a considerable distortion of the directionalcharacteristic at another frequency.

Let us, therefore, assume that at the particular frequency at whichantenna of Fig. 3 is to be operated, stay 3i] has a low self impedanceso that it picks up and scatters a considerable amount of power whichwould normally be radiated by the antenna.

Under these circumstances the induced cur rent in the stay will bedistributed in the form of standing waves. This distribution of currentis diagrammatically indicated in Fig. 3 by the dotted wavy line i.

Since the position of current maxima and minima with respect to the endsof a stay depends on the relation of terminal impedances to the surgeimpedance of the stay, the distance of the first current maximum fromone of the ends may have any value between minus A, wavelength and pluswavelength depending on the terminal impedance. When a stay isterminated by a good insulator the first current maximum occursapproximately at A; wavelength from the insulated end. When, as in Fig.3, a stay is terminated by a very large metal object such as funnel 34,without insulation, the first current maximum occurs but a smallfraction of a wavelength from the end of the stay. For this reason if weimagine that stay 30 were cut at a point J the impedance of the shortposition of the stay between J and funnel 34 as seen at J wouldgradually increase to a maximum as J is moved from 34 toward a point A;wave from 34. This maximum impedance seen at J when looking into J-34 isusually of the order of 1500 ohms. If then at J, which is located at afraction of a wavelength from a current maximum, there were connected anauxiliary wire 31 there would be directed a substantial portion of thecurrent into this auxiliary wire. The shelf impedance of the whole staywould, of course, change and the effect produced by varying the lengthof 3| would be approximately the same as the effect which would beproduced by altering the length of stay 30 when 3| is disconnected.Thus, the self impedance of the stay may be controlled by varying thelength of the auxiliary wire 3|. By controlling the self impedance inthis manner the current in the stay may be reduced to a small fractionof what it is when the stay has a very low self impedance. The wire 3|may be terminated by an insulator 32, or may be connected directly toground or to a large metal object. In the latter case the wire 3| shouldpreferably be about A; wavelength shorter or longer than when it isterminated at an insulator.

It sometimes happens that after a stay has been detuned at onefrequency, it acquires a low self impedance at another frequency atwhich it may be desired to use the antenna. In such a case an additionaldegree of freedom is required. Fig. 4 illustrates how this additionaldegree of freedom may be secured in practice. In this figure 3| and 35are two auxiliary wires connected at two points J1 and J 2 to stay 30.By adjusting the length of these wires one can detune stay 30 at twofrequencies. The points of junction of the auxiliary wires with the staymay either incide or be separated by a substantial distance.

In Fig. 4 there is shown still another arrangement for the same purpose.This: arrangement consists of the auxiliary wire 36 to which there isconnected another auxiliary wire 31. By adjusting the length of wires 36and 3'! as well as the points of junction J3 and J4 it is possible todetune stay 33 at two different frequencies. All of the auxiliary wiresin Fig. 4 may be either terminated by insulators or connected to largemasses of metal depending upon which of the various possiblearrangements results in greater detuning effect and is simpler to erect.

In Fig. 5 another method is illustrated for re ducing the undesirableeffects of a stay. This method is particularly useful when. the antennawith which the stay is interfering is a receiving antenna.

It has already been pointed out that a high resistance stay hasrelatively little effect on the directional characteristic of an antennaand that it can merely affect the total radiated or received power.Since the signal to static ratio is not affected when the gain of anantenna is reduced equally in all directions but is usually seriouslyaffected when the shape of radiation characteristic is distorted, it isclear that the presence of a high resistance stay in the neighborhood ofa receiving antenna is not nearly as objectionable as is the presence ofav low resistance radiating stay.

In Fig. 5 is shown an arrangement which enables one to convert a lowresistance stay into a high resistance stay. The arrangement in itssimplest form consists of an auxiliary wire 50 and a resistor 5!. Thevalue of resistor 5| is not critical but it should be of the same orderof magnitude as the surge impedance of the stay and of the auxiliarywire 50. The length of the auxiliary wire 50 is likewise not critical.This length may be a small fraction of the wavelength or a whole wavelength or even longer depending on mechanical convenience. The distancebetween the point of junction of auxiliary wire 50 and stay 30 and theend of the stay, however, should be chosen with reasonable care. Theproper position of junction J is fixed by the following considerations.If at some frequency F, point J falls at a current maximum along stay 30the addition of wire 50 and resistor 5| has little effect and littlecurrent is diverted into 50 under these circumstances. On the contrarywhen point J falls at a current minimum the major portion of the staycurrent is diverted into wire 50 and hence into resistor 5|. In thislatter case a large portion of the power in the stay is dissipated inresistor 5| and the stay acts as a high resistance stay. When point J isa small fraction of a wave length from the current minimum the effect ofnetwork 50, 5! is still quite considerable. For this reason the effectof this network is not confined to a single frequency but is spread outover a band of frequencies. In fact by using two such networks, one oneach end of the stay, it is often possible substantially to suppress theeffects of a stay within the whole range of frequencies received by anaperiodic antenna.

So far we have assumed that there is in the vicinity of the stay somelarge metal object to which resistor 5| may be connected. When this isnot the case either an auxiliary counterpoise may be constructed forthis purpose or else resistor 5| may be placed between wire 50 and thepoint of junction J. In this latter case wire 5!] will function as agood counterpoise as long as it is roughly A; wavelength long and isinsulated at its far end.

While I have described particular embodiments of my invention forpurposes of illustration, it should be understood that variousmodifications and adaptations thereof, occurring to one skilled in theart, may be made within the spirit of the invention as set forth in theappended claims.

What is claimed is:

1. A bi-directional antenna comprising two substantially parallelradiant acting conductors spaced a short distance apart, each of saidconductors being at least about a wavelength of the operating frequencylong, inclined at an angle to the horizontal and having their upper endsconnected together, a transmission line connected to one of the lowerends and an impedance equal to the surge impedance of the antennaconnected between the other of the lower ends and ground wherebytraveling waves are caused to fiow in opposite directions in said twoconductors so as to render the radiant action pattern of the completeantenna bidirectional.

2. A bi-directional antenna comprising two substantially parallelradiant acting conductors spaced a short distance apart, and eacharranged in the form of an inverted V, the ends of said conductors atone side of the V being connected together and the ends of saidconductors at the other side of the V being connected to a transmissionline, and through an impedanceto ground, respectively.

3. A- bi-directional antenna comprising two parallel radiant actingconductors of a length in the order of at least a wavelength at theoperating frequency of said antenna inclined with respect to thehorizontal at an angle so related to the length of said conductors thatthe radiant action pattern of each of said conductors is effected in thedesired direction and connected together at adjacent ends, atransmission line connected to one of the remaining ends and means forsuppressing reflections connected to the other of the remaining endswhereby a bi-directional radiant action pattern is produced for thecomplete antenna by the addition of the unidirectional patterns ofradiant action of all the conductors comprised in said antenna.

4. A lei-directional antenna comprising two parallel radiant actingconductors of a length in the order of at least a wavelength at theoperating frequency of said antenna inclined with respect to thehorizontal at an angle so related to the length of said conductors thatthe radiant action pattern of each of said conductors is efiected alongthe desired direction and having their upper ends connected together, atransmission line connected to one of the lower ends and means forsuppressing reflections connected to the other of the lower ends wherebythe radiant action pattern of the whole antenna is bi-directional.

5. A bi-directional antenna comprising two parallel radiant actingconductors each arranged in the form of a. V, the ends of saidconductors at one side of the V being connected together and the ends ofthe conductors at the other side of the V being connected to atransmission line and to means for suppressing reflections of wavestraveling along the antenna, respectively.

6. A bi-directional radiant acting system comprising a wive translator,a terminating impedance, a plurality of substantially parallel radiantacting conductors each of a length in the order of at least a wavelength at the operating frequency of said antenna serially connectedbetween said wave translator and said terminating impedance the saidconductor connected to saidterminating impedance having a surgeimpedance substantially equal to said terminating impedance, at leasttwo of said radiant acting conductors being inclined with respect to thehorizontal at angles so related to their length and elevation aboveground, that the radiant action patterns thereof are substantiallyunidirectional for waves traveling-in one direction therealong, and allsaid conductors being so oriented and spaced that the radiant actionpattern of the whole system. is predominantly bi-directional.

7. A lei-directional radiant acting system comprising a wave translator,a terminating impedance, a plurality of substantially parallel radiantacting conductors each in the order of at least a wavelength longserially connected between said wave translator and said terminatingimpedance, said conductor connected to said terminating impedance havinga surge impedance substantially equal to said terminating impedance, atleast two of said radiant acting conductors being inclined with respectto the horizontal at angles related to their lengths substantially inaccordance with a smooth curve defined by the appended tabulatedrelationship, and all said conductors being so oriented and spaced thatthe radiant action pattern of the whole system is predominantlybi-directional Length in wave- 1 lengths 1 2 3 4 5 6 Angle ofinclination 47 34 27 24 22 20 ANDREW ALFORD.

